Biodegradable Polymers—Biomedical Devices and Drug Delivery Systems
Traditionally, pharmaceuticals have primarily consisted of small molecules that are dispensed orally (as solid pills and liquids) or as injectables. Over the past three decades, however, sustained release formulations (i.e., compositions that control the rate of drug delivery and allows delivery of the therapeutic agent at the site where it is needed) have become increasingly common and complex. Nevertheless, many questions and challenges regarding the development of new treatments as well as the mechanisms with which to administer them remain to be addressed.
Although considerable research efforts in this area have led to significant advances, drug delivery methods/systems that have been developed over the years and are currently used, still exhibit specific problems that require some investigating. For example, many drugs exhibit limited or otherwise reduced potencies and therapeutic effects because of they are generally subject to partial degradation before they reach a desired target in the body. Once administered, sustained release medications deliver treatment continuously, e.g. for days or weeks, rather than for a short period of time (hours or minutes). Furthermore, orally administered therapeutics are generally preferable over injectable medications, which are often more expensive and are more challenging to administer, and thus it would be highly desirable if injectable medications could simply be dosed orally. However, this goal cannot be achieved until methods are developed to safely shepherd drugs through tissue barriers, such as epithelial or dermal barriers, or specific areas of the body, such as the stomach, where low pH can degrade or destroy a medication, or through an area where healthy tissue might be adversely affected.
One objective in the field of drug delivery systems, therefore, is to deliver medications intact to specifically targeted areas of the body through a system that can control the rate and time of administration of the therapeutic agent by means of either a physiological or chemical trigger. Over the past decade, materials such as polymeric microspheres, polymer micelles, soluble polymers and hydrogel-type materials have been shown to be effective in enhancing drug targeting specificity, lowering systemic drug toxicity, improving treatment absorption rates, and providing protection for pharmaceuticals against biochemical degradation, and thus have shown great potential for use in biomedical applications, particularly as components of drug delivery devices.
The design and engineering of biomedical polymers (e.g. polymers for use under physiological conditions) are generally subject to specific and stringent requirements. In particular, such polymeric materials must be compatible with the biological milieu in which they will be used, which often means that they show certain characteristics of hydrophilicity. They also have to demonstrate adequate biodegradability (i.e., they degrade to low molecular weight species. The polymer fragments are in turn metabolized in the body or excreted, leaving no trace).
Biodegradability is typically accomplished by synthesizing or using polymers that have hydrolytically unstable linkages in the backbone. The most common chemical functional groups with this characteristic are esters, anhydrides, orthoesters, and amides. Chemical hydrolysis of the hydrolytically unstable backbone is the prevailing mechanism for the polymer's degradation. Biodegradable polymers can be either natural or synthetic. Synthetic polymers commonly used in medical applications and biomedical research include polyethyleneglycol (pharmacokinetics and immune response modifier), polyvinyl alcohol (drug carrier), and poly(hydroxypropylmetacrylamide) (drug carrier). In addition, natural polymers are also used in biomedical applications. For instance, dextran, hydroxyethylstarch, albumin and partially hydrolyzed proteins find use in applications ranging from plasma substitute, to radiopharmaceutical to parenteral nutrition. In general, synthetic polymers may offer greater advantages than natural materials in that they can be tailored to give a wider range of properties and more predictable lot-to-lot uniformity than can materials from natural sources. Synthetic polymers also represent a more reliable source of raw materials, one free from concerns of infection or immunogenicity. Methods of preparing polymeric materials are well known in the art. However, synthetic methods that successfully lead to the preparation of polymeric materials that exhibit adequate biodegradability, biocompatibility, hydrophilicity and minimal toxicity for biomedical use are scarce. The restricted number and variety of biopolymers currently available attest to this.
Therefore a need exists in the biomedical field for non-toxic, biodegradable, biocompatible, hydrophilic polymers, which overcome or minimize the above-referenced problems. Such polymers would find use in several applications, including components for biomedical preparations, pharmaceutical formulations, medical devices, implants, and the packaging/delivery of therapeutic, diagnostic and prophylatic agents.
Chromatographic Applications:
Another important aspect pertaining to polymeric materials is that of chiral polymers for use as chiral chromatographic phases for the separation of stereoisomers.
The separation of mixtures of stereoisomers (enantiomers or diastereomers) into individual optical isomers is one of the most challenging problems in analytical chemistry, reflecting practical considerations important in many areas of science, particularly the pharmaceutical and agricultural industries.
For example, the pharmaceutically active site of many drugs is “chiral,” meaning that the active site is not identical to a mirror image of the site. However, many pharmaceutical formulations marketed today are racemic mixtures of the desired compound and its “mirror image.” The separation of racemates of active compounds into their optical antipodes has gained increasing importance in recent years, since it has been demonstrated that the enantiomers of a chiral active compound often differ significantly in their actions and side-effects. One optical form (or enantiomer) of a racemic mixture may be medicinally useful, while the other optical form may be inert or even harmful, as has been reported to be the case for thalidomide.
Accordingly, chiral drugs are now extensively evaluated prior to large scale manufacturing, both to examine their efficacy, and to minimize undesirable effects attributable to one enantiomer or to the interaction of enantiomers in a racemic mixture. The United States Food and Drug Administration has recently issued new regulations governing the marketing of chiral drugs.
Early chiral separation methods used naturally occurring chiral species in otherwise standard separation protocols. For example, natural chiral polymeric adsorbents such as cellulose, other polysaccharides, and wool were used as early as the 1920's. Later strategies used other proteins and naturally occurring chiral materials. These early strategies gave some degree of success. However, the poor mechanical and chromatographic properties of naturally occurring materials often complicated the separations. Although naturally occurring chiral materials continue to be used for chiral separations, efforts have increasingly turned to synthesizing chiral materials having better mechanical and chromatographic properties.
Separating optical isomers often requires considerable time, effort, and expense, even when state-of-the-art chiral separation techniques are used. There is a continuing and growing need for improved chiral separation techniques, as well as new compositions and methods useful in chiral separations of enantiomeric mixtures.
Chiral Compounds Synthesis:
Many biologically active molecules are optically active (chiral), and usually the biological activity can vary greatly depending on the optical purity of the molecule. As mentioned above, in pharmaceutical applications, the optical activity can have a great impact the activity of the drug, and thus on its marketability. Much research activity has been focused on the development of technologies allowing access to pure enantiomers.
Typically, pure enantiomers may be obtained by one of three methods: (1) chiral synthesis, (2) achiral synthesis followed by indirect resolution, or (3) achiral synthesis followed by resolution by chromatography.
The ability to build optical activity into the molecule as it is being synthesized is an important asset, and significant research efforts have been devoted to the development of enantioselective syntheses in recent years. Chiral synthesis requires a chiral starting point, it is complex and requires care to avoid racemization. The chiral purity must be monitored throughout the synthesis. The advantages are apparent in the long term due to the lack of wastage of the unwanted enantiomer and the ability to scale up the reaction to production size. However, enantioselective syntheses are often difficult, time consuming, and require chiral reagents that are generally expensive.
Chiral compounds may also be obtained from achiral synthesis (which is generally faster, more accessible and significantly less costly than a chiral synthesis), by subjecting the achiral material to indirect resolution methods such as crystallization, enzymatic reaction (which selectively destroys the unwanted isomer), or diastereoisomeric resolution (formation of the diastereomer followed by crystallization). The major drawback with these methods is that they all require a chiral selector with a very high degree of enantioselectivity, which implies that it must be very pure itself. An impure selector will result in a loss of purity and yield of the enantiomers resolved.
Alternatively, the separation of enantiomers may be achieved by chromatographic techniques, either by using chiral stationary phases (CSP) or chiral mobile phase additives (CMPA) to perform the chromatographic separation, or by forming a diastereomeric derivative suitable for chromatographic separation. Nevertheless, these methods suffer from the same disadvantages as the methods outlined above: they are time consuming, require chiral reagents/stationary phase that are generally expensive, and the chromatography can also be difficult and may take considerable development.
Therefore, a need exists for novel materials and methods that would effectively and inexpensively allow access to useful chiral compounds.